WiFi can be thought of as "wireless Ethernet" and is a standard way of connecting PDA's and other computers to local area networks.  All cards listed below are based on the 802.11b wireless standard unless otherwise noted and are SD unless otherwise noted.  Secure Digital has become one of the most popular memory forms in recent years. SD Bluetooth cards can work in only an SDIO slot, not simply SD. Quotes are from the respective companies webpage and are not a value judgment of davespda.com.
 

Sandisk Connect
"The Wi-Fi SD card is the smallest Wi-Fi card in the market. The card will allow SDIO enabled devices to connect to HotSpots (802.11b Access Points) worldwide for fast wireless Internet connectivity. The card supports Pocket PC 2002, Pocket PC 2003, Pocket PC 2003 Mobile Edition and Palm Zire 71 devices that have an SDIO enabled slot."

Sandisk Connect Plus 256
"SanDisk?s 256MB + Wi-Fi SD card combines flash memory and wireless communications into a single card giving SDIO-enabled PDA the freedom to quickly transmit and receive data, images and music from thousands of 802.11b hotspots worldwide."

Socket WL6200
"Now you can access enterprise and public Wi-Fi (IEEE 802.11b) Wireless LAN systems from your Pocket PC 2002 and 2003 using the smallest and lightest form factor available with Socket?s Low Power SDIO Wireless LAN Card."

Feel free to email me about your own PDA, questions, comments, articles, rumors, and reviews.  I can be reached at dave@davespda.com.